1. Field of Invention
The present invention relates to a double diffused metal oxide semiconductor (DMOS) device and a method of manufacturing the DMOS device; particularly, it relates to a DMOS device which has a gate having a ring-shaped structure, and a method of manufacturing the DMOS device.
2. Description of Related Art
FIGS. 1A-1C show a top view and two cross-section views of a conventional DMOS device, respectively. Referring to FIGS. 1A-1C, an isolation structure 12 (including multiple cross-sectional regions from cross-section views) is formed in a P-type silicon substrate 11 to define a first device region 100 and a second device region 200. The isolation structure 12 for example is formed by local oxidation of silicon (LOCOS). A gate 13 having a ring-shaped structure is formed on the substrate 11. A body region 14, a lightly doped drain 15, a body electrode 16, and a source 17 are formed in the first device region 100, and a drain 18 is formed in the second device region 200. In normal operation of the DMOS device, a channel is formed between the source 17 and the drain 18. However, at the edges of the channel, in particular around the corners of the ring-shaped structure of the gate 13, the body region has a relatively lower concentration of the P-type impurities, and thus the impurities are diffused to a relatively smaller distance; in contrast, the lightly doped drain 15 which is doped with the N-type impurities has about similar concentrations at the corners of the ring-shaped structure and in the middle of the channel. Therefore, when the DMOS device operates, the resistance around the corners of the ring-shaped structure of the gate 13 is relatively lower, such that the characteristics of the DMOS device are adversely impacted as shown in FIGS. 2A and 2B. FIG. 2A shows the relationship between the gate voltage Vg and the drain current Id in logarithmic scale. FIG. 2B shows the relationship between the gate voltage Vg and the drain conductance gm. From FIGS. 2A and 2B, it can be seen that due to the relatively lower resistance around the corners of the ring-shaped structure, in real condition, the DMOS device will turn ON at relatively lower gate voltage Vg as indicated by the solid line, as compared to the ideal condition indicated by the dash line. That is, the performance of the DMOS device is deteriorated in the real condition by the aforementioned corner effect.
More specifically, FIG. 1C is a cross-section view taken along the cross-section line AA′ in FIG. 1A, which shows the middle of the channel; and FIG. 1B is a cross-section view taken along the cross-section line BB′ in FIG. 1A, which shows the edge of the channel. As shown in FIG. 1B by the distance between the body region 14 and the gate 13, because the body region 14 has a relatively lower concentration around the corners of the ring-shaped structure, the impurities are diffused to a relatively smaller distance. In contrast, as shown in FIG. 1C by the distance between the body region 14 and the gate 13, the impurities concentration in the middle of the channel are relatively higher, and the body region 14 is diffused to a larger distance. The relative locations of the body region 14 and the gate 13 in FIGS. 1B and 1C indicate that the N-type impurities of the lightly doped drain 15 are less counter-doped by P-type impurities in FIG. 1B (at the edges of the channel, in particular around the corners of the ring-shaped structure) than in FIG. 1C (middle of the channel). Hence, the resistance around the corners of the ring-shaped structure is relatively lower and the sub-threshold voltage there is also relatively lower. In consequence, the DMOS device will turn ON at a relatively lower voltage as shown in FIGS. 2A and 2B, i.e., the threshold voltage of the DMOS device is reduced.
In view of the foregoing, the present invention provides a DMOS device and a method of manufacturing the DMOS device, to improve the drawback that the DMOS device turns ON at a lower threshold voltage, and to increase the threshold voltage of the DMOS device; the present invention improves the parameters of the devices at the corners of the ring-shaped structure, such that the DMOS device has abetter performance.